> I am trying to protect my HF RF amplifier from static coming down the
>> feed
>> line, and would like to bounce some ideas off a few actual EE's. I
>> currently have a neon bulb going from the center conductor of my coax to
>> ground, but this is far from satisfactory. It does nothing for charges
>> under the rated 125V. One person suggested putting a couple of
>> back-to-back
>> diodes in parrallel with the neon bulb, and I had the idea of putting a
>> 100K
>> multi-watt resistor in series with these diodes.
>>
>> This would (I believe) protect the input to the amplifier by taking
>> anything
>> over .6V to ground, and slowing the dissipation of a large pulse to
>> protect
>> the diodes somewhat and possibly even cause the neon bulb to light up (an
>> indication of a less-than-ideal environment in which to operate a radio.)
>>
>> Does this sound like an effective means of protecting said equipment from
>> incidental static charges following the coax in through my window? I
>> have
>> already had a .22uF 50V polyethylene capacitor linking the coax to my
>> tuning
>> capacitor blown by static, and replaced it with a .1uF 630V capacitor.
>> This
>> "fuse" has so far remained intact, but I would like to augment it some
>> and
>> further protect the project I spent so much time putting together and
>> troubleshooting. Anyone with any ideas is welcome to comment, cuss or
>> discuss. I am not an EE, and am open to any and all feedback.
>>
>> Thanks,
>>
>> Dave
>
> There are quite a bunch of protection devices available. There are
> small gas discharge gaps (much like your neon lamp, but specifically
> rated for very low inductance and relatively low firing voltage and
> fast firing). There are some low capacitance ESD protection chips in
> 0603 SMT packages that seem to do a good job. And I can recommend
> also using RF PIN diodes back-biased to a level a bit above the peak
> RF voltage you expect from your antenna under normal operating
> conditions. This last solution can be implemented with one PIN diode,
> anode to the antenna terminal and cathode to a low-inductance
> capacitor to ground, with the capacitor biased through a resistance to
> a zener diode. You might consider a couple 0.1uF 50V (or better,
> 200V) ceramics in 0805 SMT packages in parallel for the capacitance,
> and about 100 ohms off to +10V. Duplicate that with reverse
> polarities for a -10V clamp. Use robust PIN diodes; for HF at 50 ohms
> nominal, PINs with reverse-biased capacitance under 1pF should be
> fine. This sort of clamp should not degrade the receiver's IIP3
> materially; there are a couple significant advantages to using PINs
> instead of fast switching diodes in this application.
>
> It can also be a help to put a very simple bandpass filter between the
> antenna input terminal and the clamp (and the receiver input):
> assuming a 50 ohm input resistance for the receiver, just a capacitor
> sized to give a 3dB corner with 50 ohms at the lowest frequency of
> interest, in series with an inductor sized to give a 3dB corner
> somewhat above the highest frequency of interest.
>
> Cheers,
> Tom
>
Hello Tom,
Wow. Thank you for your post, and the ideas/suggestions you offer. I have to ask however, what is a PIN diode? And what is the IIP3 of a receiver? I am sorry I have to ask, but I have been out of the loop for over 20 years, and am just now trying to get back into electronics. And I know next to nothing about RF. I cannot tell you how much I appreciate your post however, and hope I don't frustrate you with my questions.
Best regards,
Dave